A seat belt pre-tensioner is installed in a vehicle to wind in slack in a seat belt which is being worn by an occupant of the vehicle in the event that there is an anticipated crash situation. The pre-tensioner pulls the seat belt tight against the occupant to minimize movement of the occupant relative to the seat belt if or when the crash situation occurs.
In most cases, a seat belt pre-tensioner is activated in response to a signal from a pre-crash sensor which detects that a crash situation may be about to occur. When the seat belt pre-tensioner is activated it is important that the pre-tensioner winds in the slack in the seat belt quickly so that the seat belt is tensioned correctly against the occupant, ready for the crash situation.
In some instances it may be necessary for a seat belt pre-tensioner to wind in a seat belt with sufficient force to re-position an occupant wearing the seat belt. For instance, it may be necessary for the pre-tensioner to wind in the seat belt with sufficient force so that the occupant is pulled back against the seat into a preferred sitting position. The occupant can thus be repositioned to be restrained in a safe manner by the seat belt during a crash situation.
Conventional seat belt pre-tensioners are usually powered by the main power supply in a vehicle which is normally a 12V battery. The current which can be supplied by a vehicle battery is unfortunately limited and the current limit can place undesirable constraints on the operation of a seat belt pre-tensioner. In some cases, the current limit prevents the motor in the seat belt pre-tensioner from rotating with sufficient speed to tension the seat belt quickly. In other cases, the current is not high enough to allow the motor to exert sufficient force on the seat belt to re-position an occupant.
It has been proposed previously to set the ratio of the gearing which links the pre-tensioner motor to the spool of the safety belt so that the motor rotates the spool with a sufficiently high speed. However, setting the gear ratio in this way lowers the force exerted on the seat belt which is undesirable. Alternatively, variable gear box arrangements have been proposed to provide a high-speed gear ratio during the wind-in phase and a lower-speed but higher torque gear ratio during an occupant re-positioning phase. However, variable gearbox arrangements are expensive and difficult to implement.
The present invention seeks to provide an improved drive arrangement.